Implement tail for docker logs

Fixes #4330
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)

tailfile/tailfile.go

@@ -0,0 +1,61 @@
+package tailfile
+
+import (
+	"bytes"
+	"errors"
+	"os"
+)
+
+const blockSize = 1024
+
+var eol = []byte("\n")
+var ErrNonPositiveLinesNumber = errors.New("Lines number must be positive")
+
+//TailFile returns last n lines of file f
+func TailFile(f *os.File, n int) ([][]byte, error) {
+	if n <= 0 {
+		return nil, ErrNonPositiveLinesNumber
+	}
+	size, err := f.Seek(0, os.SEEK_END)
+	if err != nil {
+		return nil, err
+	}
+	block := -1
+	var data []byte
+	var cnt int
+	for {
+		var b []byte
+		step := int64(block * blockSize)
+		left := size + step // how many bytes to beginning
+		if left < 0 {
+			if _, err := f.Seek(0, os.SEEK_SET); err != nil {
+				return nil, err
+			}
+			b = make([]byte, blockSize+left)
+			if _, err := f.Read(b); err != nil {
+				return nil, err
+			}
+			data = append(b, data...)
+			break
+		} else {
+			b = make([]byte, blockSize)
+			if _, err := f.Seek(step, os.SEEK_END); err != nil {
+				return nil, err
+			}
+			if _, err := f.Read(b); err != nil {
+				return nil, err
+			}
+			data = append(b, data...)
+		}
+		cnt += bytes.Count(b, eol)
+		if cnt > n {
+			break
+		}
+		block--
+	}
+	lines := bytes.Split(data, eol)
+	if n < len(lines) {
+		return lines[len(lines)-n-1 : len(lines)-1], nil
+	}
+	return lines[:len(lines)-1], nil
+}